Secretory proteins produced in polarized epithelial cells are targeted to either the basolateral or apical domains of the plasma membrane by either constitutive or regulated pathways, or both. The intracellular routing of a secretory protein is probably dependent on interactions(s) between components of the secretory apparatus and the secretory protein itself, but the signal(s) required to accomplish this is (are) unknown. The objective of this research is to develop a system in which it will be possible to examine the nature of those signals and the means by which those pathways are controlled. The properties of two polarized epithelial cell lines will be investigated: MDCK cells, possessing only the constitutive pathway: bovine uterine epithelial (BUE) cells, whose apical pathway may be regulated. The presence of a regulated pathway in BUE cells will be determined by electron microscopic analysis and susceptibility to secretagogues. It has been shown that laminin, a basement membrane protein, is secreted basolaterally and constitutively by MDCK and BUE cells. Since no apically secreted proteins from these cells have been described, expression plasmids containing the genes for ovalbumin, human growth hormone, or rat pancreatic ribonuclease will be introduced into the cells. Synthesis of these proteins will be monitored by immunoprecipitation of [35S]methionine labelled cells, and/or by use of a solid-phase radioimmunoassay. The proteins encoded by the exogenous genes are secreted via regulated pathways in their cells of origin. Determination of how and where each is sorted in a foreign cell, i.e., via either a constitutive or regulated pathway, and to either the basolateral or apical surface or both, will furnish information on how protein traffic is controlled.